Field of the Invention
The invention relates generally to the field of measuring nuclear magnetic resonance properties of porous media or biological tissues. More particularly, the invention presents a method of low frequency NMR relaxometry to acquire total amount of hydrogen in a sample containing a constituent with fast spin-spin NMR relaxation. The fast relaxation constituent could be, for example, kerogen in a core or drill cuttings samples of earth formations or protein in biological samples.
Background Art
NMR relaxation measurements use a static magnetic field to align nuclei in a sample with the direction of the static magnetic field to achieve a thermal equilibrium state characterized by a bulk nuclear magnetization. The rate at which the bulk magnetization is established is described by a spin-lattice relaxation (also called longitudinal relaxation) characterized by a time constant T1. A RF magnetic field orthogonal to the static magnetic field is typically used to disturb the equilibrium state to produce precession of the nuclear magnetization about the static magnetic field. The RF magnetic field is typically applied in a form of short pulses that produce free induction decay signals in an NMR antenna. The decay of the nuclear magnetization in the plane perpendicular to the static magnetic field is associated with a spin-spin relaxation (also called transversal relaxation) characterized by a time constant T2. If the static magnetic field is in Z-direction of Cartesian coordinates, then the transversal component of the nuclear magnetization is in X-Y plane (rotating due to precessional motion of the nuclear magnetization). The spin precession induces in an induction coil—a typical NMR antenna—a sinusoidal signal due to precession of the bulk nuclear magnetization about the static magnetic field with characteristic resonance or Larmor frequency corresponding to the static magnetic field strength. In order for an NMR signal to be induced in the induction coil the coil is adapted to have its sensitivity direction in the X-Y plane. The signal in the NMR antenna is proportional to the density of protons present in the sample. The bulk nuclear magnetization in X-Y plane decays due to reversible (caused by an inhomogeneity of the static magnetic field) and irreversible (true transversal relaxation) processes of de-phasing. The reversibly de-phased spins can be re-phased using refocusing RF pulses, in particular in a form of a standard CPMG sequence.
Acquiring fast spin-spin relaxation components of the NMR signal in the NMR relaxometry is typically limited by the “dead-time” of the measurements. The “dead-time” is typically determined by the RF pulse width and the after-pulse ringing time. Both limiting factors cause the “dead-time” to be inversely proportional to the NMR frequency. Typically, for low field NMR the “dead-time” can be made as short as 0.05 ms. This make the low field NMR measurements well suitable for acquiring NMR relaxation signals from the hydrogen nuclei in liquid constituents of a sample. The liquid constituents typically do not have NMR relaxation times shorter than 0.2 ms. Low field (low Larmor frequency) NMR relaxomentry has been successfully used to characterize porous space and fluids in the earth formations (e.g., U.S. Pat. No. 4,717,878, U.S. Pat. No. 5,055,787, and U.S. Pat. No. 6,452,388) as well as other samples including porous samples and biological tissues (e.g., U.S. Pat. No. 6,882,147 and U.S. Pat. No. 7,366,559). It has not been used for analyzing substances containing constituents with spin-spin relaxation times in the microsecond range, for example, for acquiring signature and the total amount of hydrogen in kerogen, or in protein molecules.
Thus known in the art low field NMR relaxometry is not suitable for NMR measurements when the measurement samples contain extremely fast relaxation components, for example, rock samples containing kerogen or biological tissues containing protein. Therefore it is an objective of the present invention to provide a solution for NMR characterization of samples having fast transversal NMR relaxation using low field NMR relaxometry.